The present invention relates to a semiconductor laser apparatus widely employed in optical devices such as optical communications apparatus, optical data processing apparatus, or optical instruments, and, more particularly to a semiconductor laser apparatus of a structure which does not induce noise even if reflected light re-enters the same semi-conductor laser.
The semiconductor laser has recently been used in an increasing number of applications, typical examples of which are pickups for optical discs and an optical data transmission terminals.
In a terminal for optical data transmission, for example, the driving current of a semiconductor laser is modulated by data signals to be transmitted, the output light from the laser is focussed by a lens and is coupled to an optical fiber which is a transmission line for the optical signals. In a pickup for an optical disc such as an optical video disc, semiconductor laser beam is directed onto the surface of the optical disc through a plurality of lenses in a finely focused spot to follow the signal track. The light reflected from the recording pit of the optical disc surface is deflected in the direction of the optical path by a half mirror or a polarizing prism inserted between collimating and focusing lenses or between a laser diode and a collimating lens, and then detected by light detectors.
With these optical devices, optical noise problems have occurred in that the light reflected from optical components in the transmission path returns to the semiconductor laser to cause instability of oscillation modes, thereby generating noise. In an optical data transmission terminal, for instance, the light emitted from a semiconductor laser is reflected from both sides of a LD-fiber coupling lens, or the optical fiber ends and returns to the semiconductor laser again. In a pickup of an optical video disc, the light reflected from a disc substrate as well as from optical members provided in the optical path returns and enters the semiconductor laser and causes noise as described above. Noise deteriorates the quality of optical data transmission, and of images reproduced by the disc.
When the S/N of the image reproduced by a video disc is assumed to be 45 dB, the permissible noise level required for a semiconductor laser ranges from -135 dB/Hz to -140 dB/Hz. When there is no return light, the noise level is maintained in the range from -140 to -150 dB/Hz with a strong single axial mode, but if there is return light in 0.1%-1%, the level deteriorates to -110 to -120 dB/Hz accompanied with a decline in the pictorial noise. image S/N to 42 dB, thus remarkably deteriorating the pictorial image quality.
A simple optical isolator comprising the combination of a polarization beam splitter and a quarter-wave length plate is often used in a pickup of conventional optical video discs in order to restrict the return of the reflected light. The principle of the above isolator lies in that the light which has gone through and come out from the quarter-wave length plate perpendicularly intersects the polarized light before it enters said plate and the return light is deflected and eliminated by the polarization beam splitter. However, isolation obtained by such an isolator is limited to the range of 20-25 dB at most even when the reflection at the optical disc plane is not accompanied by a change in polarizing conditions. 0.3-1% of light is always returned to the semiconductor even though the transmittance of a collimating lens which collimates the beam from the semiconductor laser is maintained at 30%. As is well known, the surface of an optical disc is protected with a resin film. As such protective films are not always uniform in refractive index birefringence, the polarized plane of the reflected light becomes rotated.
A polarization beam splitter can therefore not completely eliminate the return light, and the light returning to the semiconductor laser increases to thereby enhance noise.
In order to reduce noise generation due to the axial mode instability of the semiconductor laser, there has been proposed a method of superimposing a high-frequency current of 500 MHz-1 GHz on a DC driving current of the semiconductor laser to reduce the coherence of the laser light even if there is ca. 0.1-1% return light. (See, for example, Nikkei Electronics, 1983, 10, 10, pp. 173-194 (in Japanese)). This method, however, is detrimental as it requires an additional circuit for superposing high frequency current as well as a sophisticated technique for packaging the circuit with the semiconductor laser because the high-frequency circuit should be matched with the semiconductor laser in impedance, the cost becomes inevitably high.
There is known another method for reducing noise which uses a semiconductor laser having oscillation characteristics to cause oscillation of 1 GHz on the emitted light simply by applying a DC current instead of superimposing the high-frequency signals on the driving current. (S. Matsui et al, "Suppression of feedback-induced noise in short cavity V-channeled substrate inner stripe lasers with self-oscillation", Appl. Phys. Lett. 43 (3), Aug. 1 1983, pp. 219-221). The mechanism of such self-oscillation has not been elucidated yet. Effective oscillation characteristics cannot be obtained unless the composition and thickness of the active layer of a semiconductor laser as well as those of the cladding layer adjacent to and between said layer and the substrate are controlled to remain within a closely restricted range. The currently available technique of growing crystal in the semiconductor laser manufacturing process does not have sufficient control over these factors nor reasonable yield, resulting in a high price.
The return of light to a semiconductor may be prevented by inserting in the optical path an optical isolator as an element which has a non-reversible transmission characteristic. Light wavelength of a semiconductor laser employed in an optical video disc, etc. is in the range of 0.78-0.85 micron. Isolators to be employed in the above wavelength range pass the light beam through high density lead glass inserted in an intensive magnetic field (See, for example, M. Saki et al, "Optical Isolators for Fiber-Optic Communications", Paper of Technical Group, TG OQE 78-133, pp. 25-30, IECE of Japan (in Japanese).). However, such an isolator is as large as a match box has considerable weight and costs a great deal. Moreover, such an isolator requires a powerful permanent-magnet to provide a high magnetic field, which inevitably causes considerable magnetic leakage. An isolator of this type cannot therefore be suitably used in a pickup for an optical disc or in optical communication devices.
As described in the above, the prior art methods have drawbacks in one way or another.